The present invention relates generally to microfabricated porous membranes with bulk support, and more particularly to microfabricated porous membranes whose bulk support is an unetched portion of a substrate on which the membrane was fabricated.
Porous membranes may be used as elements of absolute particle filters, immunological barrier capsules and time-release diffusion barriers. Devices incorporating porous membranes must have sufficient mechanical strength to withstand routine handling during fabrication and use. These devices must also be able to withstand the mechanical stresses inherent in their intended use.
Filtration devices are an indespensable necessity, for example, of current health care technology, and in particular of biotechnology. Within the health care industry, examples of areas that require accurate filtration devices are patient and blood product protection, diagnostics, and the fields of pharmaceutical, biotechnology and bioprocessing, including blood fragmentation technology. For several applications within these domains, required filter features include: control of the pore sizes and distributions, and of the relative diffusion rates; absolute pore sizes as small as the nanometer range; high durability; and biochemical and mechanical resistance. Certain existing, commercially-significant filter technology employs polymeric membranes, and is incapable of addressing a variety of needs in the cited application areas.
Precise control of filter pore sizes in the 50 to 100 angstrom range, for either organic or inorganic filters, would allow, for example, biologically important molecules to be mechanically separated on the basis of size. In the present state of the art, there is a very limited selection of filters having pores much less than the resolution limit of 0.35 microns of photolithography. Some filters known heretofore having pore sizes in this range include polycarbonate membrane filters, sintered filters, zeolites, and one instance of a microfabricated bulk micromachined filter.
Polycarbonate membrane filters (nucleopore filters) may be used where pore sizes between 500 and 3500 angstroms are needed. These filters, however, cannot be used at high temperatures, in strong organic solvents, or where no extracted oligomers can be tolerated. The pores of polycarbonate membrane filters are also randomly located. As such, there is a compromise between having a high enough population of pores per unit area and having too many instances of partially overlapping pores. Partially overlapping pores provide pathways through the filter that allow some particles to get through that are larger in diameter than the rated cut-off size of the filter.
Filters that are available in other materials, such as metals or ceramics, are made by sintering together discrete particles. This technique yields a random structure with a relatively large dead volume and no exact cut-off size above which transport is impossible.
Materials such as zeolites, which have a crystal structure with large channels, can be used as molecular sieves in the limited range of from about 5 to 50 angstroms. Zeolites are not amenable to fabrication as thin membranes.
A microfabricated filter with bulk micromachined structures is described by Kittilsland in Sensors and Actuators, A21-A23 (1990) pp. 904-907. This design uses a special property of silicon wherein silicon becomes resistant to a certain etchant when highly doped with boron. The pore length of this filter is determined by the lateral diffusion of boron from a surface source at a single crystal silicon-thermal oxide interface. As is known, such lateral diffusion is the diffusion in the plane of the source, away from the source. The use of this technique makes it very difficult to precisely control the pore length. Also the method of fabricating this filter cannot be applied to materials other than silicon.
Thus, it can be seen that currently there is a need for filters that can be fabricated from a wide variety of materials and that have pores of well controlled shape and size smaller than about 3500 angstroms and arranged in a precise pattern to exclude the possibility of overlap.
Such pore sizes are also necessary for microencapsulation for immunological isolation. Medical researchers have demonstrated that the concept of microencapsulation to provide immunological isolation is valid. The islets of Langerhans, which produce insulin in mammals as well as hormones that control the metabolism of glucose by other organs, have been transplanted between different species. For example, pig islets have been transplanted into diabetic dogs to control glucose metabolism. However, these unprotected islets function only for a short time before the immune system of the host kills the donor cells.
Encapsulation of islets in order to protect them from immune system macromolecules has been shown to prolong the survival of donor cells. By using various means of encapsulation, insulin production from pig islets has been maintained for over one hundred days in dogs. Encapsulation methods to date include using semipermeable amorphous organic polymeric membranes, sintered together particles, and intermeshed ceramic needles. Significant problems have been encountered, however, limiting the useful life of these capsules to not much more than one hundred days.
The two principal problems with the capsules described above are inadequate mechanical strength and insufficient control of pore size and pore distribution. Specifically, if the thickness of an organic membrane capsule wall is increased to provide the required mechanical strength, molecules cannot diffuse through the capsule wall quickly enough to provide the appropriate physiological response when needed. Moreover, if the size and distribution of pores cannot be controlled, such as with sintered together particles or amorphous polymeric membranes, there is a high probability of oversized or overlapping pores which could provide an opening large enough for immunological macromolecules to enter the capsule.
It is desirable that a capsule combine mechanical strength with the ability to allow the free diffusion of small molecules such as oxygen, water, carbon dioxide, and glucose, while preventing the passage of larger molecules such as the immunoglobins and major histocompatibility (MHC) antigens. Also, the intermediate sized molecular products, such as insulin, produced by the donor cells should be able to diffuse out to the host at a sufficient rate to provide the needed metabolic function.
Accordingly, it is an object of the present invention to provide a structure combining mechanical strength with controlled pore size down to the nanometer range.
Another object of the present invention is to provide such a structure obtainable using simple fabrication techniques.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims.